1. Field of the Invention
The present invention generally relates to systems and methods for controlling operation of a center pivot irrigation system and, more particularly, to a center pivot irrigation system with a corner arm that is configured to provide, with the irrigator's corner arm nozzles (e.g., drop nozzles), a variable (or user-selectable) depth of water application in differing zones of land irrigated by the corner arm (e.g., variable rate irrigation (VRI) under the corner arm).
2. Relevant Background
In many areas of the world, the agricultural industry relies upon effective irrigation to raise crops. Irrigation may be used due to limited rainfall or rainfall that is not adequate for a particular crop. Further, irrigation may be utilized due to the variability in the timing of rainfall as many crops require a relatively consistent amount of water over their growing period.
Center pivot irrigation is one of the most water-efficient irrigation techniques. Center pivot irrigation is a form of overhead sprinkler irrigation that makes use of a long sprinkler arm formed of several segments of pipe joined together and supported by trusses. The pipe segments are mounted on wheeled towers each typically driven by an electric motor. Sprinklers or sprinkler heads are spaced apart along the length or span of the sprinkler arm. The sprinkler arm is centrally anchored such that the sprinkler arm moves in a circular pattern, and water is fed into the sprinkler arm at the center of the circle via one, two, or more pumps that may be positioned anywhere between the sprinkler arm and a water source such as an aquifer.
The outside set of wheels on the arm may set the master pace for the rotation such as once every three days. The inner sets of wheels can be mounted at hubs or towers between two pipe segments, and angle sensors can be used to detect when the bend at the joint between the two segments exceeds a certain maximum threshold as an indication of when the electric motor driving the inner wheel sets should operate to rotate these wheels to keep the pipe segments generally aligned along the arm span. Center pivot sprinkler arms are typically less than about 1600 feet (or 500 meters) in length (i.e., irrigated circle radius) with a common size being 1320 feet (or a quarter mile or 400 meters) length or circle radius, but many pivot sprinkler arms may be much longer than 500 meters in length.
Most center pivot irrigation system use sprinklers that hang down from the sprinkler arm pipe segments so that the sprinkler heads are positioned a few feet above the ground or crop so as to limit evaporative losses and wind drift. There are many different sprinkler head or nozzle configurations that may be used including a static plate, a moving plate, and so on with a rotator-style pivot applicator sprinkler head or nozzle being one of the more popular in present irrigation systems. The rotator-style sprinkler head or nozzle is adapted to work properly with input water at a particular pressure (or within a desired range and not very well or at all if too far below this pressure), and a pressure regulator may be provided upstream of each nozzle or sprinkler head to ensure that each is operating at the correct design pressure (not over pressurized).
It is typically an operational goal of a center pivot irrigation system to provide uniform application of water, which may include effluent, chemicals, and fertilizers, and to avoid applying too much water in one application (e.g., by running too slowly) so as to reduce run-off and leaching. To achieve such uniform application, the center pivot irrigation system generally requires an even emitter flow rate across the length or span of the sprinkler arm. Since the outer-most pipe segments and wheeled towers travel farther in a given time period (e.g., at a higher speed) than the inner-most pipe segments and wheeled towers, nozzle sizes may be smallest at the inner spans and increase in size with distance from the pivot point such that a greater flow of water is output at positions on the sprinkler arm that is moving the fastest (and a lower flow of water is output at positions on the sprinkler arm that is moving the slowest).
While center pivot irrigation systems are considered to be highly efficient systems that are useful in conserving water, there remains a number of challenges associated with their use to irrigate crops. One challenge is that center pivot irrigation systems may also include a corner sprinkler arm (or corner arm extension system) extending outward from the last regular drive of the pivot sprinkler arm, and it can be difficult to provide uniform application of water from the corner sprinkler arm. The last regular drive and/or tower provides the final set of drive wheels at the outer most point of the pivot sprinkler arm (e.g., a point in the pipe section or span that is most distal to the center point or rotation axis), and the corner sprinkler arm (or corner arm section) is a section of pipe that has a steerable set of wheels enabling it to trail behind the main pivot pipe or pivot sprinkler arm or to swing out so as to extend the effective length (or radius) of the center pivot irrigation system. The use of corner sprinkler arms is desirable as it enables the irrigation system to irrigate into the corners of a field (e.g., irrigate a more square geographic area) rather than just irrigating a circular portion of the field.
The use of a corner sprinkler arm in a center pivot irrigation system can be problematic because the corner sprinkler arm typically will not provide very good uniformity of depth of application during irrigation operations. As discussed above, uniform depth is important to farmers to achieve consistency of crop growth across their irrigated fields. Non-uniformity results because as the corner sprinkler arm is opening up (its outer end is swung out relative to the last regular drive), the end of the corner sprinkler arm at the outermost extent of irrigation must travel at a speed that is much faster than the last regular drive. Also, as the corner sprinkler arm is falling back behind the main part of the pivot sprinkler arm, the outer end of the corner sprinkler arm must travel at a speed that is much slower than the last regular drive of the pivot sprinkler arm. Conversely, the opposite is true when the irrigation system is running in a reverse direction.
The speed that nozzles on the corner sprinkler arm travel over the ground is directly correlated with the depth of the application of water. For example, a nozzle will apply less water to the ground when the corner sprinkler arm is opening up and traveling faster when compared with the same nozzle traveling more slowly as it is falling back or is trailing behind the last regular drive of the pivot sprinkler arm. To further complicate the problem of non-uniformity of application, the position of nozzles differs between irrigation systems designed and/or manufactured by different companies as does the actual path the corner arm follows during irrigation processes. As a result, each nozzle of a corner sprinkler arm of each irrigator system can have its own unique acceleration and deceleration profile. Another problem with providing uniform application with a corner sprinkler arm is that the same piece of land (or portion of an irrigated field) may be affected by two or more nozzles passing over it, with each at the same or different flow rates and/or speeds.
Center pivot irrigation system manufacturers have attempted to address the non-uniformity problem associated with corner sprinkler arms by turning off banks of nozzles depending on the bearing or present location of the pivot sprinkler arm in a field. For example, banks of four nozzles each may be turned off in the corner sprinkler arm. However, this and other approaches to improve control of the irrigation system have not proven effective and do not tend to provide good uniformity (or acceptable uniformity) in water application by center pivot irrigation systems with corner sprinkler arms. Hence, there remains a need for an improved control method for operating such center pivot irrigation systems (and irrigation systems incorporating such a control method) to provide more uniform application of water (and other liquids/materials such as fertilizer) on portions of fields watered or affected by the corner sprinkler arm.